Try using a battery charger. It works for me. See:
http://a12mopar.com/yabb/YaBB.pl?num=1366043257A battery charger is going to have more amps than D cells, so that may be your problem. Even a battery charger is capable of plating only a relatively small area. I use 4 zinc electrodes around my bucket since plating only happens on the side of the part facing the electrode. That's why zinc plated tubing is not plated on the inside.
I think the sugar mentioned in this link should actually be Karo syrup. You can try either and none and see what happens.
The health risks with hexavalent chromium in the Caswell sodium dichromate are extremely high. Follow all safety precautions such as rubber gloves and face shield/mask, don't breathe any fumes, don't get any on your skin, and do proper disposal.
You can get more yellow and rainbow effect cheaper than the Caswell by buying 1 lb of sodium dichromate powder on ebay for under $17 shipped. Mix 372 grams of powder with 1 gallon of distilled water and add 34 ml of 35% sulfuric acid. You can get battery acid (should be 35% sulfuric acid) from most car parts stores for $10 or less. This is called a Cronak solution (Google it). Dip your zinc plated part in the Cronak for 15 seconds, immediately rinse in a bucket of clean water, and dry with a heat gun. In case of Caswell, dip for about 2 minutes. Dip your part on the end of some wire, of course.
